EP0467913B1

EP0467913B1 - Method and apparatus for recovering heat from solid material separated from gasification or combustion processes

Info

Publication number: EP0467913B1
Application number: EP90905526A
Authority: EP
Inventors: Juhani Isaksson; Harry Ollila
Original assignee: Ahlstrom Corp
Current assignee: Ahlstrom Corp
Priority date: 1989-04-13
Filing date: 1990-04-11
Publication date: 1993-02-10
Anticipated expiration: 2010-04-11
Also published as: JPH0765735B2; FI891748A; EP0467913A1; FI891748A0; FI86219C; KR920701757A; JPH04501168A; KR950012569B1; US5624469A; ES2039127T3; FI86219B; WO1990012253A1

Abstract

A method and apparatus for recovering heat from solid material, such as ash, removed from hot processes such as combustion or gasification processes and/or from cleaning processes of hot gases. The ash is conducted from a combustion chamber (2) or from a gas cleaning means (16) into a heat exchanger (8), where it is brought into heat exchange contact with liquid for transferring heat from the ash to the liquid. The liquid heated in this manner is mixed with the feed material of the hot process, such as for example coal, in a mixing chamber (14) for moistening and preheating the feed material prior to feeding thereof into the combustion chamber (2).

Description

The present invention relates to a method and apparatus for recovering heat from solid material discharged from hot processes such as combustion or gasification processes and/or from cleaning processes of hot gases. In particular, the invention relates to the re-use of recovered heat in hot processes, and more specifically, it is suitable for recovering heat from the ashes of gasification or combustion processes.
Handling of ashes derived from combustion and gasification processes is problematic. For one thing, the hot ash has to be cooled before storing, and for another, spreading of ash and especially the finest ash particles after cooling causes environmental hazards.
Efforts have been made to get ride of fine ash by agglomerating it by means of heating it either in a combustion chamber or in a separate agglomerating means, whereby the ash released from the process becomes more applicable to storage.
It has also been suggested to use ash for drying of fuel. For example, Swedish patent application 8501563-4 discloses mixing of hot ash with fuel prior to feeding the fuel into a combustion chamber. In this case, the moisture possibly contained in the fuel is either absorbed in the ash or evaporates. The fuel becomes drier and it is easier to handle in the equipment constructed for treating conventional dry material. At the same time, however, the quantity of ash circulating in the process becomes greater, which is less desirable in terms of energy economy.
Efforts have also been made to cool and agglomerate ash by mixing water therewith in order to receive ash which is more appopriate for storage, such as is disclosed in DE patent specification 34 01 847. In this disclosure, the cooling water constitutes a problem because the temperature of the water is low, it is difficult to utilize its heat. Depending on the method of cooling and amounts of water, the temperature of the cooling water of ash is generally below 200°C. The water is too cold to be added in the steam circuit. On the other hand, the temperature of the cooling water is not much different from the temperature of the boiler water. Therefore, the cooling water cannot be used for heating of the boiler water with means of reasonable size. Discharging of water and not utilizing its heat is not a good solution either in terms of heat economy.
It is known from US patent 4, 244, 706 a gasification process where both gaseous and solid products from the gasification are simultaneously cooled by direct heat exchange with water in a char cooling device. Thus cooled product gas and vaporized water are separated from the solids and further cooled in order to condensate water and organic materials such as phenols and other aromatics, which are to be recycled. Cooling is accomplished by indirect heat transfer in a heat exchanger. The cooled condensate is mixed with fresh make-up water and thereafter mixed with coal in a mixing tank. The resulting mixture is preheated before being introduced into the gasifier.
It is known through US patent 4, 111, 158 a method to cool bed material discharged from a fluidized bed reactor in order to control the bed temperature in the fluidized bed reactor. The bed material is discharged from the reactor into a heat exchanger where the material is cooled indirectly. The cooled bed material is thereafter recycled into the fluidized bed reactor.
The solids required for gasification and combustion processes have to be fed undisturbed into the combustion chamber and in such a manner that the various substances are distributed as evenly as possible over the cross sectional area of the entire combustion chamber. Feeding of dry, fine material, for example, into a pressurized combustion chamber calls for rather complicated equipment. Conveyance of dry material in the pipework prior to feeding it into the combustion chamber is both energy-consuming and difficult. For example, transfer of line coal causes dusting and the fine coal itself is clearly explosive.
To avoid the above-mentioned drawbacks, moistening of coal prior to feeding thereof into the combustion chamber has been suggested. Finnish patent application 865217 discloses a method of mixing water with slack. Water is mixed with the slack to such an extent as to form a pumpable mass. This coal paste can be pumped with a pump without any additional treatment directly to the combustion stage.
An object of the present invention is to provide a method of recovering heat from solids discharges from hot processes, which methods is superior in terms of the heat recovery to the methods described above. Another object of the invention is to provide an advantageous method of bringing the discharged solids into a form suitable for further treatment.
A still further object of the present invention is to provide an improved method of treating the feed material of the gasification or combustion process prior to feeding such feed material to a combustion or gasification reactor.
To gain the objects described above, the method of the invention, in which heat is recovered from hot solids separated from hot processes, includes following steps

solid feed material, such as solid fuel or additive for the process, and fluidizing gas, such as air is fed into the the combustion chamber,
process gas formed in the combustion chamber is discharged from the upper part of the combustion chamber and
hot solid material, such as ash formed in the combustion chamber or other solid bed material being fluidized in the combustion chamber is
- discharged from the bottom part of the combustion chamber, through discharge means or
- separated from process gas and discharged from the process in a hot gas cleaning means,
and conducted via a channel into a heat exchanger, where the hot solid material is brought into indirect heat exchange contact with water flowing through a conduit in the heat exchanger for transferring heat from the hot solid material into the water, thereby providing hot water,
hot water, thus formed in indirect heat exchange contact with substantially only discharged hot solid material, is conveyed uncooled from the heat exchanger through a conduit into a mixing chamber and mixed with solid feed material in the mixing chamber;
the solid feed material is simultaneously heated and moistened by mixing in with the hot water in the mixing chamber and
the heated and moistened solid feed material is conveyed from the mixing chamber through a conduit into the combustion chamber.

The apparatus for recovering heat in accordance with the invention comprises

a solid feed material conduit for feeding solids into the combustion chamber,
fluidizing gas openings in the bottom grate of the combustion chamber for fluidizing solid material in the combustion chamber,
a discharge pipe in the upper part of the combustion chamber, for discharging process gas from the combustion chamber,
discharge means in the bottom part of the combustion chamber for discharging hot solid material from the combustion chamber and/or discharge means for discharging hot solid material separated from the hot process gases in a hot gas cleaning means,
a heat exchanger connected to the discharge means for receiving the discharge hot solid material,
a conduit in the heat exchanger for leading water through the heat exchanger in indirect heat exchange contact with the discharged hot solid material and for transferring heat from the hot solid material to the water,
means for removing cooled solid material from the heat exchanger,
a mixing chamber connected to the solid feed material conduit,
means for introducing solid feed material into the mixing chamber and
a conduit connecting the heat exchanger directly to the mixing chamber for feeding hot uncooled water from the heat exchanger into the mixing chamber for moisturing and heating the solid feed material in the mixing chamber.

The present invention is suitable for recovering heat from the ashes of gasification or combustion processes, such as fly ash being continuously separated in the gas cleaning means. The invention is also suitable for recovering heat from bed material discharged from the combustion chambers of fluidized bed reactors. The bed material generally contains mainly ash and inert solid material such as sand. The bed material may also contain some additive, such as sulphur-binding calcium compounds, fed into the process. Bed material is removed either continuously or intermittently from the reactor for maintaining suitable process conditions in the reactor. Because the material to be discharged is hot, it usually has be cooled prior to further treatment thereof.
In the method of the invention, the fluid used as a heat exchange medium is preferably water. Mixed with the feed material, it is well applicable to be fed into a hot process. Depending on the process and temperatures, some other fluid may also be used.
In accordance with the invention, hot solid material can be introduced either into an open fluid tank such as, for example, a water tank disposed below the combustion chamber of the reactor, or into a closed water tank. In a closed water tank, hot solid material effects vaporization of water, and it can led forward in the evaporated form. Vapor is readily transported even over long distances. When vapor is brought into contact with colder feed material such as, for example, slack, it will condense onto the surface of the coal particles thus heating the coal and forming a coal paste suitable for feeding thereof into the combustion chamber.
Liquid is according to the invention brought into an indirect heat exchange contact with removed solid material.
When the invention is applied in respect of gasifiers or boilers in which coal is gasified or combusted, heated liquid or possibly steam is mixed with coal, preferably to such an extent as to produce coal paste which is easy to treat an handle. Increasing the moisture of coal to 15-50 % facilitates its transportation by pumping even longer distances and enables simples feeding thereof even into a pressurized combustion chamber. Feeding can be effected by fairly simple means. Raising of the moisture content prevents dusting of coal and considerably reduces its susceptibility to explode.
The invention is further described in more detail below, by way of example, with reference to the accompanying drawings, in which

Fig. 1 is a shematic illustration of an embodiment of the invention, and
Fig. 2 is a shematic illustration of a second embodiment of the invention.

The embodiment according to Fig. 1 is applied to heat recovery from ash which has been removed from the pressurized combustion chamber 2 of a fluidized bed reactor 1, and to heat recovery from other possible bed material. In this embodiment, water is used as the medium of heat recovery. The heated water is further utilized for adjusting the temperature and composition of the fuel in the fluidized bed reactor.
A fluidized bed is maintained in the combustion chamber 2 of the fluidized bed reactor 1 by introducing fluidizing gas, such as air, through openings in a bottom grid 4 into the combustion chamber. Fluidizing gas can, of course, be fed also by other generally used air feed means such as air nozzles, disposed at the bottom of the combustion chamber. The process gas is removed from the upper part of the combustion chamber by a gas discharge pipe 5. Fuel such as, for example, coal is introduced into the combustion chamber through a conduit 3. The combustion chamber can be arranged with several fuel feeding conduits if necessary. Furthermore, the combustion chamber can be provided with various conduits, not shown in the figure, for introducing bed material or additive into the process.
Ash and other solid material possibly discharged from the bed are removed from the bottom part of the combustion chamber by a discharge means 6. The ash discharged from the combustion chamber is conducted via channel 7 into a heat exchanger 8, which comprises a chamber 9. In the chamber, in the embodiment according to the Fig. 1, the ash is brought into indirect heat exchange contact with liquid flowing through conduit 10, which liquid may be, for example, water. The cooled ash is conveyed from chamber 9 through conduit 11 and through a pressure reduction valve to a storage tank 12.
The water heated in the heat exchanger is introduced through a conduit 13 into a mixing chamber 14, where water is mixed with fuel 15. The fuel may be pulverized coal or slack. Heated water is mixed with the fuel, preferably to such an extent that the moisture content of the fuel will rise to 15 - 20 %, whereby an easily pumpable paste is formed by, for example, coal. The mixing chamber is connected to the fluidized bed reactor by means of the conduit 3.
The invention is applicable to both unpressurized and pressurized combustion or gasification processes. Feeding of ash from a pressurized combustion chamber can be simply arranged in a pressurized state into a pressurized heat exchanger. When cooled, the ash easily be led through the pressure reduction valve into a storage tank 12.
In some applications, it may be preferable to locate the pressure reduction valve between the combustion chamber and the heat exchanger and to arrange the cooling of ash at atmospheric pressure.
In the method according to Fig. 2, the invention is applied to recovering heat from fly ash. The gas from the fluidized bed reactor 2 is cleaned in means 16. The fly ash is introduced via conduit 7 to the heat exchanger 8. As in the previous embodiment, the heat of the ash is recovered in a liquid, which is introduced to the mixing chamber 14 for moistening and preheating the fuel to be fed into the process.
In particular, the heat contained in the fly ash has been difficult to utilize because the amounts of fly ash are small and the heat content of fly ash is lower than, for example, that of the bottom ash of the reactor. The method according to the invention now makes possible also the utilization of the heat of the fly ash.

Claims

A method for recovering heat from hot solids being discharged from combustion chambers of fluidized bed reactors in combustion or gasification processes, in which
- solid feed material, such as solid fuel or additive for the process, and fluidizing gas, such as air is fed into the combustion chamber,

- process gas formed in the combustion chamber is discharged from the upper part of the combustion chamber and

- hot solid material, such as ash formed in the combustion chamber or other solid bed material being fluidized in the combustion chamber is
- discharged from the bottom part of the combustion chamber, through discharge means or

- separated from process gas and discharged from the process in a hot gas cleaning means,
and conducted via a channel into a heat exchanger, where the hot solid material is brought into indirect heat exchange contact with water flowing through a conduit in the heat exchanger for transferring heat from the hot solid material into the water, thereby providing hot water,

- hot water, thus formed in indirect heat exchange contact with substantially only discharged hot solid material, is conveyed uncooled from the heat exchanger through a conduit into a mixing chamber and mixed with solid feed material in the mixing chamber;

- the solid feed material is simultaneously heated and moistened by mixing it with the hot water in the mixing chamber and

- the heated and moistened solid feed material is conveyed from the mixing chamber through a conduit into the combustion chamber.
A method as claimed in claim 1, characterized in that
- ash is discharged from the bottom part of a pressurized fluidized bed reactor and brought into indirect heat exchange contact with water in a heat exchanger and

- water heated in the heat exchanger is mixed with solid feed material for increasing the moisture in the feed material to 15 - 50% for enabling the feed material to be pumped into the pressurized fluidized bed reactor.
A method as claimed in claim 1, characterized in that ,
- fly ash separated from the process gas in a pressurized fluidized bed reactor is brought into indirect heat exchange contact with water in a heat exchanger and

- water heated in the heat exchanger is mixed with pulverized coal to produce a coal paste with a moisture content of 15 - 50 % befor feeding it into the pressurized fluidized bed reactor.
A method as claimed in claim 1, characterized in that
- solid material is led from a pressurized fluidized bed reactor into a pressurized heat exchanger, where the solid material is brought into indirect heat exchange contact with water.
An apparatus for recovering heat from hot solids being discharged from fluidized bed reactors in combustion or gasification processes, comprising a combustion chamber in a fluidized bed reactor, having
- a solid feed material conduit (3) for feeding solids into the combustion chamber (2),

- fluidizing gas openings in the bottom grate (4) of the combustion chamber for fluidizing solid material in the combustion chamber,

- a discharge pipe (5) in the upper part of the combustion chamber, for discharging process gas from the combustion chamber,

- discharge means (6) in the bottom part of the combustion chamber for discharging hot solid material from the combustion chamber and/or discharge means (16) for discharging hot solid material separated from the hot process gases in a hot gas cleaning means,

- a heat exchanger (8) connected to the discharge means (6, 16) for receiving the discharged hot solid material,

- a conduit (10) in the heat exchanger for leading water through the heat exchanger in indirect heat exchange contact with the discharged hot solid material and for transferring heat from the hot solid material to the water,

- means (11) for removing cooled solid material from the heat exchanger,

- a mixing chamber (14) connected to the solid feed material conduit (3),

- means for introducing solid feed material (15) into the mixing chamber and

- a conduit (13) connecting the heat exchanger directly to the mixing chamber for feeding hot uncooled water from the heat exchanger into the mixing chamber for moisturing and heating the solid feed material in the mixing chamber.